This invention relates to methods and apparatus for generating highly luminous high temperature, low NO.sub.x flames using fluid fuels. The methods and apparatus disclosed may be used as the major source of energy and also as an assisting energy source in melting furnaces, industrial heating and heat treating furnaces, kilns, incinerators and other high temperature applications.
Industrial furnaces for high temperature heating (above 1800.degree. F.) and melting applications commonly use gaseous fuels such as natural gas. When ambient air (especially preheated air) is used as the oxidizer the gaseous flames are normally blue with very low luminosity thus reducing the heat exchange between the flame and the furnace contents. By comparison, luminous oil flames result, in many cases, with as much as 25% less fuel consumption and a decrease in exhaust gas temperatures.
Utilization of oxygen to increase the heating efficiency due to higher flame temperature and the lower volume of flue gases is a well-known combustion approach. However, known methods of introduction of the oxygen into a combustion process through the injection of oxygen into the combustion air, commonly called oxygen enrichment, result in even further reduction of flame luminosity and an increase of NO.sub.x emissions due to oxidation of nitrogen with oxygen inside the higher temperature flame. The ratio control between fuel flow and enriched air flow is similar to standard fuel/air ratio control. The ratio of total air to total oxygen is normally constant during the heating cycle.
U.S. Pat. No. 3,729,285 to Schwedersky discloses a staged method of gaseous fuel combustion inside the burner tunnel wherein two fuel streams with associated air streams are separately controlled, delivered to the combustion chamber and burned prior to mixing the combustion products together to create a low NO.sub.x flame. In order to be able to accomplish stable pyrolysis with air, having approximately 78% N.sub.2, inside the burner, the ratio of oxidizer to fuel would have to be at least 75% of stoichiometric to insure adequate heat release during the partial combustion of the fuel being pyrolyzed, since heat stored in the N.sub.2 molecules in the pyrolysis zone will be unavailable for pyrolysis. This diluted flame core reduces flame temperature inside said core, and also has a relatively low air deficiency, eliminating the possibility of significant formation of carbon microparticles of the proper size required to provide adequate flame luminosity. Thus, this invention cannot practically utilize pyrolysis, since most fuel is used to create heat, leaving little fuel to be pyrolyzed.
Carbon microparticles required for maximum emissivity are in the size range of 0.05 to 4.0 microns and the formation of particles in this size range has been shown to be optimized at above 3000.degree. F., which is a temperature that is not achievable without a high concentration of oxygen in the oxidizing gas. In the low temperature conditions of Schwedersky excessive hydrocarbon fuels will generate predominately more hydrocarbon radicals than particles of carbon capable of boosting flame luminosity. Also a low flame core temperature significantly reduces the radiative capability of said particles. The flame introduced by the Schwederskypatent has less NO.sub.x emissions than other flames, but it also has low radiative heat flux due to the low temperature of the flame envelope.
Because the ratio of total oxygen to total fuel is not variable in such systems, the volume of flue gases per Btu at standard conditions is fixed throughout the entire firing cycle. The temperature inside the refractory combustion chamber is slightly reduced due to staged combustion. The Schwedersky Patent also describes a burner utilizing more than one oxidizing stream to stage combustion, but does not have the ability to utilize pure oxygen, as is made clear by the fact that metal as used in the combustion chamber could not survive the temperature of a flame utilizing pure oxygen, nor does it disclose enrichment by or use of pure oxygen.